Prep the loaf: Preheat the oven to 375°F (190°C). Line a baking sheet with foil. Slice the top third off the loaf lengthwise and hollow out the bottom, leaving about a ¾-inch wall so it stays sturdy. Save the top; you’ll use it later.

Toast the shell: Brush the inside of the hollowed loaf with 1 tablespoon melted butter. Place both the bottom and the top on the baking sheet, cut-sides up. Bake for 5–7 minutes to lightly crisp. This helps prevent sogginess.

Sauté the veggies: Heat olive oil and 1 tablespoon butter in a large skillet over medium-high heat. Add onions and peppers. Cook 5–7 minutes until softened and lightly golden. Add mushrooms and cook 3–4 minutes more. Stir in garlic for 30 seconds. Season with a pinch of salt and pepper. Remove to a bowl.

Cook the steak: Increase heat to high. Add the thinly sliced steak in a single layer, working in batches if needed. Season with salt, pepper, and smoked paprika. Cook 2–3 minutes, stirring occasionally, until just browned. Stir in Worcestershire and cook 30 seconds. Remove from heat.

Combine the filling: Return the veggies to the pan with the steak and toss to combine. Taste and adjust seasoning. You want it well seasoned since the bread will mellow the flavors.

Layer the cheese: Place a layer of provolone slices inside the toasted bread shell, covering the bottom and sides as much as possible. This creates a cheesy barrier.

Fill the loaf: Spoon the hot steak-and-veggie mixture into the loaf, pressing gently to fill evenly. Top with remaining provolone and the shredded mozzarella.

Bake until melty: Place the top of the loaf next to the filled bottom on the baking sheet. Bake for 10–12 minutes, or until the cheese is fully melted and the bread is golden.

Assemble and rest: Set the top back on the filled loaf. Let it rest 3–5 minutes so the cheese settles for easier slicing.

Slice and serve: Use a serrated knife to cut into thick slices. Garnish with chopped parsley if you like. Serve warm.